Valentin: "In regard to the diode I attached a picture. The simplest way I see is remove that jumper wire and add the diode there (it may require enlarging the holes). Then you only need to cut 1 trace and reconnect the rest of the circuit after the diode." What I would propose is to cut that trace which leads to the jumper wire just where it joins down from the external power jack and move that reverse-polarity diode to that break---that way it protects BOTH the battery and external power supplies from reverse voltage. Then put the 4 volt regulator in place of the jumper wire.
